1. Eigenvalue problems;I. Babuška,1991
2. V. P. Mikhaylov, Partial Differential Equations (Nauka, Moscow, 1983) [in Russian].
3. A. N. Tikhonov and A. A. Samarskiy, Equation of Mathematical Physics (Nauka, Moscow, 1977) [in Russian].
4. S. I. Solov’ev, ‘‘Eigenvibrations of a bar with elastically attached load,’’ Differ. Equat. 53, 409–423 (2017).
5. L. V. Andreev, A. L. Dyshko, and I. D. Pavlenko, Dynamics of Plates and Shels with Concentrated Masses (Mashinostroenie, Moscow, 1988) [in Russian].